This study contnues to employ cell fusion as a means of determining those factors acting during mitosis and leading to dissolution of a nuclear membrane and induction of prophasing on one hand and those factors which are responsible for reconstitution of the nuclear membrane on the other hand. More recently, special attention has been paid by us to the role of bivalent ions (particularly calcium) and mitochondria in the reformation of the nuclear membrane. In the future, we hope to investigate the specific role of these ions and events during glucose metabolism which are essential to the reformation of the nuclear membrane. BIBLIOGRAPHIC REFERENCES: Chai, L.S., Weinfeld H. and Sandberg, A.A.: Ultrastructural changes in te nuclear envelope during mitosis of Chinese hamster cells: A proposal mechanism of nuclear envelope formation. J. Nat. Cancer Inst. 53: 1033-1048, 1974. Obara, Y., Wenfeld, H. and Sandberg, A.A.: Detection by means of Cell Fusion of Macromolecular Synthesis Involved in the Reconstitution of the Nuclear Envelope in Mitosis. J. Cell Biol. 64, (2) 378-388, 1975.